Light shutters prepared from PLZT or like material having an electrooptical effect are driven usually by the arrangement shown in FIG. 1. The illustrated light shutter 1 has electrodes 2 and 3 at its opposite sides. With the electrode 2 grounded, a pulse voltage of suitable intensity is applied to the other electrode 3 as a drive voltage Vd1, whereby light incident on the light shutter 1 through a polarizer 4 is polarized within the shutter 1 to pass through an analyzer 5. The light is modulated depending on whether the drive voltage Vd1 is thus applied to the shutter 1.
FIG. 2 shows the relationship between the voltage applied to the light shutter of FIG. 1 and the intensity of light transmitted through the analyzer. With such light shutters, it is known that the intensity of transmitted light reaches a maximum level when a specific voltage is applied at which the angle of polarization of the light within the shutter is in match with the analyzer in the static characteristics, that is, usually when a half-wave voltage V.lambda./2 specific to the shutter 1 is applied to polarize the light at 90.degree..
To drive the light shutter 1, therefore, the half-wave voltage V.lambda./2 is conventionally applied to the other electrode 3.
In the case where the light shutter is used in electrophotographic printers or the like, the shutter must be driven with pulse in conformity with the speed of rotation of the photosensitive drum for forming images thereon. However, if it is attempted to drive the shutter with the half-wave voltage V.lambda./2 in the form of pulses to give transmitted light of maximum intensity, there arises the problem that the photosensitive drum can not be fully exposed to light and produces only a poor contrast if the pulses are of short duration. For a full exposure of the photosensitive drum, the half-wave voltage V.lambda./2 needs to be applied to the shutter in pulses of increased duration or width, but this entails the problem of greatly reducing the system speed of the printer or the like and failing to realize a high-speed operation.
On the other hand, when an array of a multiplicity of such light shutters arranged in a line is used, for example, as the writing head of an electrophotographic apparatus including a photosensitive member, with half-wave voltage V.lambda./2 applied to one of the electrodes of one shutter to drive the shutter, the voltage applied to the electrode is high, so that a current is likely to flow through another light shutter which is not driven, permitting light to leak through the undriven shutter to impair the contrast of the image obtained.
To overcome this problem, accordingly, we conceived utilization of the characteristics of the light shutter that with reference to the characteristics curve of FIG. 2, there is little or no transmitted light until the voltage applied to the light shutter reaches a definite level V1. For example, when the light shutter is 60 .mu.m in the distance between the electrodes at its opposite sides and 120 .mu.m in the length of its optical path and is driven with a half-wave voltage V.lambda./2 of about 50 V, there is substantially no transmitted light until the voltage reaches about 20 V. More specifically, we prepared the arrangement of FIG. 3 wherein a bias voltage Vb of up to the level V1 for giving no transmitted light was always applied negatively to the electrode 2 to be grounded, while a drive voltage Vd2, which was lower than the half-wave voltage V.lambda./2 by an amount corresponding to the bias voltage Vb, was applied to the other electrode 3 to drive the light shutter 1.
The potential difference across the electrodes 2, 3 at the opposite sides of this shutter 1 is the same as that in the foregoing case wherein the half-wave voltage V.lambda./2 is applied to one of the electrodes, such that the light shutter 1 is subjected to an electric field of the same intensity as above and affords transmitted light of the same intensity while the drive voltage Vd2 to be applied to the electrode 3 can be lower.
Nevertheless, we have found that in the case where the light shutter is thus driven, leakage of light I.sub.OFF still occurs even if the shutter is off as shown in FIG. 4.